The narrowing or constriction of vessels carrying blood to the heart is a well-known cause of heart attacks and, gone untreated, can lead to sudden death. In such stenotic vessels, it is known that the region immediately downstream from the constriction is characterized by having rapid flow velocities and/or complex flow patterns. In general, narrowing of blood carrying vessels supplying an organ will ultimately lead to compromised function of the organ in question, at best, and organ failure, at worst. Quantitative flow data can readily aid in the diagnosis and management of patients and also help in the basic understanding of disease processes. There are many techniques available for the measurement of blood flow, including imaging based methods using radiographic imaging of contrast agents, both in projection and computed tomography (CT), ultrasound, and nuclear medicine techniques. Radiographic and nuclear medicine techniques often require the use of ionizing radiation and/or contrast agents. Some methods involve making assumptions about the flow characteristics which may not necessarily be true in vivo or require knowledge about the cross-sectional area of the vessel or the flow direction.
CT is one technique of acquiring blood flow and other cardiac data. Typically, in CT imaging systems, an x-ray source emits a fan-shaped beam or cone-shaped beam toward a subject or object. Hereinafter, reference to a “subject” shall include anything capable of being imaged. The beam, after being attenuated by the subject, impinges upon an array of radiation detectors. The intensity of the attenuated beam of radiation received at the detector array is typically dependent upon the attenuation of the x-ray beam by the subject. Each detector element of the detector array produces a separate electrical signal indicative of the attenuated beam received by each detector element. The electrical signals are transmitted to a data processing system for analysis which ultimately produces an image. Generally, the x-ray source and the detector array are rotated about the gantry within an imaging plane and around the subject. X-ray sources typically include x-ray tubes, which emit the x-ray beam at a focal point. X-ray detectors typically include a collimator for collimating x-ray beams received at the detector, a scintillator for converting x-rays to light energy adjacent the collimator, and photodiodes for receiving the light energy from the adjacent scintillator and producing electrical signals therefrom. Typically, each scintillator of a scintillator array converts x-rays to light energy. Each scintillator discharges light energy to a photodiode adjacent thereto. Each photodiode detects the light energy and generates a corresponding electrical signal. The outputs of the photodiodes are then transmitted to the data processing system for image reconstruction.
Increasingly, CT imaging is being used for cardiac imaging. This increase in the implementation of CT for cardiac imaging is primarily predicated upon the relatively fast scan speeds that are now possible with CT systems and the acquisition of multi-slice data. Conventional CT systems are now capable of supporting 0.35 seconds per gantry rotation, if not faster. In fact, in the past decade, there has been nearly a three-fold improvement in temporal resolution. Multi-slice CT acquisition has also contributed to the rise of cardiac CT imaging for cardiac-related diagnoses. CT systems are now capable of significant multi-slice acquisitions.
One application of cardiac CT imaging is coronary artery imaging (CAI). The objective of CAI is to capture images and thus visualize the vasculature of the heart to detect vascular narrowing, disease, or anomalies. CAI is often used by cardiologists, radiologists, and other physicians to examine the dynamic motion of the heart muscles to detect abnormalities. To visualize the narrowing or constriction of a small vessel, the CT scan must provide high temporal resolution so as to “freeze” the cardiac motion in and around the vessel as well as provide high spatial resolution to accurately depict the size of the vessel under examination.
To improve temporal resolution, CAI studies are typically carried out with the assistance of electrocardiogram (ECG) signals acquired from the patient using an ECG monitor. An ECG monitor records the electrical activity of the heart using electrodes placed on the patient's chest, arms, and legs. An ECG monitor is commonly used to provide information regarding heart rate, heart rhythm, adequacy of blood supply to the heart, presence of a heart attack, enlargement of the heart, pericarditis, and the effects of drugs and electrolytes on the heart. ECG signals may also be used to provide cardiac phase data so as to synchronize the acquisition of CT data from the heart with the phase activity of the heart. More particularly, the CT system uses the ECG signals to consistently acquire data during the same phase of the cardiac cycle during the CT scan. Doing so reduces image artifacts.
In conventional helical CAI scans, the table translates the patient continuously at a relatively slow pace, i.e. low-pitch, to ensure that the entire heart volume is properly covered. This is illustrated in FIG. 1 where detector row position as a function of time is plotted. As shown, the cardiac cycles are separated by horizontal dotted lines 2. The detector-row locations are depicted by the solid diagonal lines 4. Every point on these lines represents a single-row projection collected at a certain z location and a particular time (therefore a particular projection angle). The z-axis extends along the length of the imaging table, as shown in FIG. 2. For simplicity of illustration, a four-row system is illustrated. The shaded boxes 6 show the reconstruction windows for the cardiac images. These boxes 6, therefore, depict a unique set of time intervals and z-locations. The width of each box 6 represents the volume in z that can be covered with reconstructions corresponding to a particular cardiac cycle. The adjacent set of reconstructions take place only after the heart reaches the same cardiac phase in the next cardiac cycle. If the combination of gantry speed and helical pitch is not properly selected, the entire heart volume will not be uniformly covered in the reconstructed images. For example, if the table travels too fast (helical pitch is too high), gaps 8 will be present between adjacent volumes. Although small gaps could be filled by image space interpolation, larger gaps will lead to discontinuities and artifacts in the volume rendered images. This is particularly problematic when considering the variation of heart rate in a typical patient.
Conventional CAI studies are typically carried out with helical pitches between 0.1 and 0.4. Such a helical pitch is commonly used to account for the worst case scenario with regard to timing, i.e. ensure complete volume coverage at specified heart rate for a given reconstruction. This translates to a higher dose to patients since regions exposed to the x-ray radiation are highly overlapped. That is, since typical helical scan x-rays are continuously projected toward the subject, these regions of overlap correspond to regions that are exposed to multiple exposures of x-ray radiation. To reduce dose to the patient during CAI studies, a number of dose reduction techniques have been developed. In one technique, the current to the x-ray tube is modulated such that the current is reduced outside the reconstruction window defined between each heartbeat. While these techniques have an advantageously reduced dose, cardiac imaging remains to be one of the highest x-ray dose applications in CT.
Therefore, it would be desirable to design an apparatus and method for cardiac CT imaging that further reduces x-ray dose as well as improves temporal and spatial resolution of CT images. It would also be desirable to have a method and system that dynamically responds to cardiac irregularities, such as arrhythmia, such that data acquisition is suspended if invalid or unacceptable data is acquired. Additionally, it would be desirable to dynamically adjust to the variation in heart rate during data acquisition.